So I have been rereading Even A Worm recently, because mostly I have been indoctrinating a friend to the Saiyuki manga (since he liked the anime and I needed to show him how many more levels of awesome the manga was, especially this arc).
I got to thinking about some of the symbolism in the characters - specifically Sanzou, Ukoku and Koumyou.
We know from Burial that Koumyou is the "moon" and Ukoku is the "night sky" and there's that kind of a rivalry between them - Burial gives the impression that Koumyou is the only one who has the power to stop Ukoku in his tracks. Whether this is to do with the Seiten Kyoumon or not is not really clear, but in the Even a Worm arc, Ukoku explains their position to one another through the recounting of the legend about the Raven and the sunspot...and the fact that the Raven and the Rabbit are two halves of the same existence.
So we know Ukoku is the raven. His name says as much, "Raven" is his apparent Western pseudonym, and the ravens are always around when he's around. The whole rabbit and the moon thing ties back to the Japanese folk idea about the rabbit that apparently lives in the moon. I think it comes from the shapes on the moon's surface, but that forms a direct tie between Koumyou and the moon and the other side of Ukoku's existence (like Yin and Yang). It's therefore interesting that the place Ukoku chooses to hide his sutra is inside a stuffed rabbit toy - a toy which he later tears up and discards before reassuming the guise of a Sanzou priest to go meddle with Hazel and attack Gokuu.
Is that rabbit symbolically Koumyou in Ukoku's mind's eye? Does it contain the sutra because Koumyou is, in some way, a restraining influence on Ukoku? Burial gives the impression that Ukoku respects Koumyou, or at least looks at him differently from any of the other individuals he encounters, including Sanzou and the Ikkou. It's Koumyou who he has the conversation about life and death and being devoured or the devourer...but they also have a strange kind of friendship, too. Even though Koumyou is dead, the moon is still there - as the Burial arc states, watching over everything.
Sanzou is, of course, the sun. However, Ukoku's raven story talks about the sunspot on the sun, the place the Raven hides. Ukoku is the harbinger of death and of nothingness. Sanzou, despite his position, has also always been surrounded by death. In a sense, then, he also has his own "sunspots" where "ravens of death" conceal themselves. It is perhaps partly for this reason that Ukoku is so interested in Sanzou. I am not sure whether Ukoku was or was not complicit in the death of Koumyou- whether he worked for Gyokumen Koushu at this time or whether that happened after. However, he knows a lot about Sanzou's birth and origins, which he almost certainly learned from Koumyou during their meetings while Sanzou was a child. Maybe a part of his intention is to bring Sanzou's darker elements out - and he almost achieves this with the attack on Gokuu. Sanzou comments internally that at this point it's the first time he's ever felt such a desire to kill someone, other than for the sake of survival. A raw killing instinct that is brought out of him by Ukoku, manipulating his feelings.
It's also important not to forget that Gokuu is also Sanzou's sun - albeit in another lifetime. In Gaiden, Konzen looks on Gokuu as his sun, rather than the other way around. Therefore it could be said that by trying to extinguish Gokuu, Ukoku is also trying to bring Sanzou into darkness. Sanzou does not have the sutra Koumyou had, that is the opposite to the sutra Ukoku holds. He only has the Maten Kyoumon, which must make him fair game.
The parallels between the deaths of Hazel's master and Sanzou's are also too similar to be coincidence. Perhaps Ukoku took in the Kamisama because he felt he needed a protege in the way Koumyou and Filbert had their respective adopted sons. However, I have another thought about this.
We know the roles that the Ikkou had in the Heaven Realm. We also know Jeep's. We do not, however, know if Ukoku had a previous existence there or not. I would not be surprised if we ultimately discovered that Ukoku was the reborn form of Nataku's father. He has the desire to create and mutate in the name of science. He has a "son" who he uses for his own ends, he tries to remove Gokuu and destroy the Ikkou, albeit for no really clear reason that I can see. Nataku's disappearance and reemergence may be nothing to do with Ukoku at all, but it seems a coincidence that he's suddenly shown up now (albeit there is a time lapse between Reload and Blast, and we don't know what Ukoku is about now).
Just food for thought, anyhow. There are so many questions still surrounding Ukoku, that I wonder if we will ever have all the answers before this story closes...